1. Field of the Invention
This invention relates a solid state laser having a wavelength conversion function, and more particularly to a wavelength conversion laser in which the wavelength of a solid state laser beam is converted by a nonlinear optical element having a periodic domain reversal structure.
2. Description of the Related Art
As disclosed, for instance, in Japanese Unexamined Patent Publication No. 62(1987)-189783, there has been known a solid state laser in which a solid state laser medium doped with a rare earth metal such as neodymium is pumped by a laser beam emitted from a semiconductor laser or the like. In such a laser, it has been in wide use to dispose a wavelength convertor such as a nonlinear optical crystal in the resonator in order to convert the laser beam to its second harmonic, thereby obtaining a laser beam having a shorter wavelength.
As the wavelength convertor, there has been known one provided with a region where the spontaneous polarization (domain) of a ferroelectric crystal having a nonlinear optical effect is periodically reversed, as disclosed, for instance, in Japanese Unexamined Patent Publication No. 5(1993)-5920. In the wavelength convertor, by setting the pitch .LAMBDA. of the domain reversals to an integer multiple of the coherence length .LAMBDA.c given by formula EQU .LAMBDA.c=2.pi./{.beta.(2.omega.)-2.beta.(.omega.)},
wherein .beta.(2.omega.) represents the propagation constant of the second harmonic and .beta.(.omega.) represents the propagation constant of the fundamental wave, phase matching between the fundamental wave and the second harmonic can be obtained.
However there has been a problem in conventional wavelength conversion lasers employing a nonlinear optical crystal having a periodic domain reversal structure that they are bad in beam quality and slow in rising.